1. Field of the Invention
This invention relates to voltage reference circuits and, more particularly, to low noise, linear temperature coefficient voltage reference circuits.
2. Description of the Related Art
Voltage reference circuits have been developed to provide precise voltage outputs for use in a variety of analog circuits such as operational amplifiers (op amps), digital-to-analog converters (DACs) and analog to digital converters (ADCs). Commonly used references include "Zener" and "bandgap", or AVBE, designs. Although such references are suitable for many applications, they are not without their problems. For example, their output voltages vary widely and nonlinearly with temperature, they are not always available in a desired voltage range, some exhibit a "hysteresis" effect, and their noise levels may preclude their use within systems which require a high degree of accuracy, especially low-power systems. Improved noise levels for both Zener and bandgap references may require operation at higher bias currents.
As an example, to attain sixteen bit accuracy over an operating temperature range of 100.degree. C. (limiting error to 1/2 least significant bit), the temperature coefficient of an ADC's voltage reference cannot exceed 0.08 ppm/.degree. C. and its noise density (for a 16 bit ADC with 10 V full scale range) must, be limited to 40 nV/.sqroot.Hz. Operating at a bias current of 100 .mu.A a Zener reference may have a noise density of 100 nV/.sqroot.Hz and a bandgap reference 300 nV/.sqroot.Hz. Improving this noise performance would require a greater operating current.
FIG.1 illustrates a basic Zener voltage reference circuit. A voltage +V.sub.s is supplied to a resistor R.sub.s that is connected in series with a reverse-biased Zener diode D1, the anode of which is connected to the anode of a forward biased diode D2, whose cathode is connected to ground. The output reference voltage V.sub.REF appearing at terminal 9, the junction of the resistor Rs and the cathode of D1, is the sum of the forward voltage drop of diode D2 and the avalanche voltage drop of diode D1. The attractive feature of this circuit is that, although the forward voltage drop of diode D2 exhibits a negative temperature coefficient, this offsets, to some degree, the positive temperature coefficient of the avalanche voltage drop of diode D1. However, since the initial temperature dependency of the diode D1 is relatively large, i.e. approximately 300 ppm/.degree. C., establishing an offsetting voltage from the diode D2 which compensates for the variation in output voltage from diode D1 over a wide operating range is somewhat difficult.
Additionally, because the avalanche breakdown voltage of diode D1 is typically in the 5 to 8 V range, the reference voltage produced by such a circuit is in the 6 to 9 V range. Since the reference must be driven from a voltage source higher than 6 V, Zener references are not suitable for operation in systems which employ 5 V or the increasingly popular lower supplies. In addition, voltage references based upon temperature compensated avalanche diodes tend to be noisy, due to noise generated by the diode's breakdown mechanism.
Band-gap references provide a temperature-compensated reference which could operate from a lower (e.g. 5 V or below) supply voltage. Band-gap references employ bipolar transistors having emitters of different sizes. Supplying the transistors with equal currents develops a difference in base-emitter voltage .DELTA.V.sub.BE between the two transistors. Such references generally produce an output of the form V.sub.BE +.DELTA.V.sub.BE (A), where A is a gain factor. The V.sub.BE and .DELTA.V.sub.BE components have opposite polarity temperature coefficients (.DELTA.V.sub.BE is proportional to absolute temperature and V.sub.BE is complemenlary to absolute temperature) which tend to cancel one another out. Numerous variations in bandgap reference circuitry have been designed and are discussed, for example, in Fink et al. Ed., Electronics Engineers' Handbook, 3d ed., McGraw-Hill Book Co., 1989, pages 8.48-8.50.
Although the output of a bandgap voltage cell is ideally independent of temperature, the outputs of bandgap cells have been found to include nonlinear temperature dependencies which are difficult to compensate. Additionally, the initial temperature dependency of the .DELTA.V.sub.BE component is quite high, approximately 3000 ppm/.degree. C., and the difficulty of compensating for a temperature coefficient is generally proportional to the magnitude of the initial temperature coefficient. Furthermore, a bandgap circuit's basic reference voltage .DELTA.V.sub.BE is developed across a fixed resistor and, because of process variations and other limits upon the accuracy with which an absolute resistance value (as opposed to a ratio of resistances) may be produced, the resistor imparts errors to the voltage reference. Amplification of .DELTA.V.sub.BE represented by the gain A, introduces further noise into the reference output. The use of an absolute resistance further degrades the bandgap reference's performance because the resistor value will drift over time, causing the reference's output to similarly drift. Yet another problem of bandgap references is a "hysteresis effect"; that is, a bandgap reference which produces an initial reference voltage will, after being heated and then returned to its initial temperature, produce a slightly different reference voltage.